It is known that most current wireless communication technologies are unable to traverse a thick solid barrier made from materials such as rock, concrete or soil. At frequencies greater than a few kHz, the skin depth of these weakly conductive materials is quite small and electromagnetic waves are attenuated to the point where communication is not possible over even short distances (less than one meter) through solid materials. In work environments such as tunnels and mines, workers are therefore not able to readily communicate, for example, with supervisors or other workers located at a different part of the tunnel or located above the surface of the earth.
Such communication systems are not only useful for regular day-to-day work; they are indispensable in times of emergency. For example, if a tunnel collapses with a worker inside, providing such communications could significantly reduce the time to find and rescue the trapped worker. This would provide a significant improvement in the chances of saving the life of the worker.
Therefore, some wireless solutions have also been proposed for through-the-earth communication based on magneto-inductive communications. These systems use a conductive wire loop to generate the transmitted magnetic field at a very low carrier frequency, typically below 10 kHz. The strength of the magnetic field created by a loop antenna is given by the antenna Dipole Moment, D, whereD=A*I*N,  [1]
A=the antenna area, in m2 
I=the antenna current, in Amps
N=the number of turns in the loop
In coal mining applications, safety limits are imposed on the current flowing through a wire of given inductance in order to prevent an open circuit from creating a spark capable of igniting an air-methane mixture. This directly affects the current, size and number of turns used in a loop antenna, and hence the dipole moment. This in turn limits the range available from a magneto-inductive communications system that is safe to use in a coal mine.
It is known to use a loop antenna for both transmit and receive functions of a through-the-earth radio. Such an antenna may comprise a number of bundles of conductors, which may be connected in parallel to form a low resistance transmit antenna, or in series to increase the number of turns in the loop for receive purposes. The antenna may have a single feed point for transmission. While the parallel conductors reduce the resistance of the coil windings they do not significantly change its inductance. The Dipole Moment of the transmit antenna configuration can therefore only be increased by increasing either the antenna size (and hence inductance) or current as with any other conventional loop antenna.
It is further known to use an underground mine communication system to effect mine-wide communication and an intrinsically safe current limiter circuit for insuring that electrical equipment in the system will not cause incendiary conditions. This current limiter ensures that the current does not exceed the limits specified by MSHA for intrinsic safety, and the transmitter Dipole Moment is thus limited as described above.
In addition, antennas for through the earth communications comprising a plurality of conducting loops where each loop may be driven by a separate transmitter have been described. The transmitters provide identical drive signals to each loop, and the loops are arranged so that their magnetic fields combine. Such an arrangement may be adapted to intrinsically safe applications by adding a current limiting circuit between each transmitter and antenna loop.
FIG. 1 is a block diagram of a conventional multi-loop antenna system. For clarity, the diagram illustrates a system with three loop antennas but other numbers (m) of antennas may be used. A VLF signal source 1 is connected to three transmit amplifiers 2a, 2b, 2c which provide an output signal that is conditioned by a matching circuit 3a, 3b, 3c and a transformer 4a, 4b, 4c to drive a current into the low impedance antenna loops 5a, 5b, 5c. A current and voltage limiter circuit, shown in dashed boxes, 6a, 6b, 6c is used between each amplifier and loop antenna to prevent the antenna current and voltage from exceeding the known safe levels for a given loop inductance. Thus if the current in one of the wires is interrupted by severing or disconnecting the wire, it will not be capable of igniting a flammable atmosphere.
If m loops are used however, the mutual coupling between the loops increases the impedance seen by the transmitter. The current in each loop can then only be maintained by increasing the transmitter voltage, which in turn imposes a lower safe current limit. To maintain the voltage and current within a safe range it is therefore necessary to limit the number of loops m to a small number, i.e. four loops, and the practical improvement in the Dipole Moment and distant magnetic B-field afforded by using multiple current limited transmit antennas is quite small.
Thus, it is clear that there remains a need to improve the performance of the transmitter for magneto inductive communications while meeting all the requirements for the intrinsic safety of the system. There is a further need for a transmitter suitable for through-the-earth radio communication using electro-magnetic induction that reduces the energy available to initiate an ignition in a flammable atmosphere when the current is interrupted.